1. Field of the Invention
The present invention relates to a permanent-magnet rotor for an inner rotor type electric rotary machine and a magnet-saving type rotor for a synchronous motor.
2. Description of the Background Art
The surface permanent-magnet motors (SPM) and the interior permanent-magnet motors (IPM) are two representative conventional synchronous machines using permanent-magnet rotors. The latter is advantageous in that mechanical support of the permanent magnets is easy and a reluctance torque can be obtained in addition to a magnetic rotor.
In the conventional IPM, permanent magnets are installed in a rotor core closely to an outer cylindrical surface thereof and disposed at equal pitches. The permanent magnets are magnetized to have alternate polarities in the circumferential direction.
Furthermore, switched reluctance motors (SRM) and synchronous reluctance motors (SYNRM) are also conventionally known as representative reluctance motors.
IPMs are widely used in industrial and automotive fields and are earnestly required, from various reasons, to reduce the size and weight as well as the manufacturing costs. Reducing the manufacturing costs will be easily attained by adopting the above-described reluctance motors (RM) or the IPMs using cheap ferrite magnets. However, the motor performance will deteriorate. The motor size will increase inevitably. This is not desirable especially for automotive motors, because the reduction in size and weight is an important factor for the automotive motors. Accordingly, adoption of the above-described reluctance motors (RM) or the ferrite magnet-based IPMs is not a preferable selection. In general, the residual magnetic flux density of ferrite magnets is small. To obtain a required amount of magnetic flux, it is definitely necessary to expand the size of the motor in the circumferential direction or in the axial direction. As a result, the motor size or weight increases inevitably.
In view of the foregoing, a preferable way to attain downsizing and weight-reduction of motors is adopting IPMs using rare-earth magnets because the rare-earth magnets have excellent residual magnetic flux density. However, the rare-earth materials are expensive. The required amount of magnets for each motor must be reduced.
To suppress the reduction of magnetic flux and to reduce the amount of magnets to be used, reducing the thickness of each magnet is effective. However, it is generally difficult to realize a thin magnet having the thickness less than approximately 2 mm from the restrictions in manufacturing processes of magnets and also from the requirements for assuring sufficient mechanical strength of magnets.
Furthermore, although the permanent magnets are installed into insertion holes of the rotor extending in the axial direction, this installation work is not easy in the case that the permanent magnets are magnetized beforehand. Accordingly, the usual procedure for installing the permanent magnets into the insertion holes is carried out by inserting non-magnetized permanent magnets into the insertion holes and then magnetizing the whole of rotor. However, according to the conventional rotors, the permanent magnets are provided near the outer cylindrical surface of the rotor core and magnetized to have alternate polarities in the circumferential direction. It is hence necessary to dispose the magnetizing magnetic poles on an inner cylindrical surface facing to the outer cylindrical surface of the rotor core so as to have alternate polarities in the circumferential direction. Accordingly, the magnetizing magnetic flux in the rotor core formed by the magnetizing magnetic poles passes the rotor core region intervening between adjacent permanent magnets and, as a result, bypasses the permanent magnets. This makes it difficult to uniformly magnetize the permanent magnets in the radial direction. Furthermore, the structure of a required magnetizing apparatus will become complicated.